1. Field of the Invention
The present invention relates to an optical transmission system and a multi-core optical fiber that can be used in the optical transmission system.
2. Description of the Related Art
In optical communications, the transmission capacity has increased rapidly with developments of an optical amplifier, a signal modulation/demodulation scheme, and the like. In addition, a demand for data has been also increasing for sure along with a spread of the fiber to the home (FTTH). Therefore, a further increase of the transmission capacity is indispensable. A method of increasing a transmission capacity is disclosed in which a holey fiber (hereinafter, referred to as “HF” as appropriate), which is a new type of optical fiber, is used as an optical transmission line. The holey fiber has a hole structure, and confines a light in the core region by holes. For example, in K. Ieda, K. Kurokawa, K. Tajima, and K. Nakajima, “Visible to infrared high-speed WDM transmission over PCF”, IEICE Electron. Express, vol. 4, no. 12, pp. 375-379 (2007), an optical transmission line with a length of 1 kilometer is deployed using a photonic-crystal fiber (PCF), which is a kind of the holey fiber, to realize an optical transmission across a broad bandwidth including wavelengths of 658 nanometers to 1556 nanometers. As for the holey fiber, some improvements have been made in terms of the length of the fiber used and the transmission loss (see, for example, K. Kurokawa, K. Tajima, K. Tsujikawa, K. Nakajima, T. Matsui, I. Sankawa, and T. Haibara, “Penalty-free dispersion-managed soliton transmission over a 100-km low-loss PCF”, J. Lightwave Technol., vol. 24, no. 1, pp. 32-37 (2006) and K. Tajima, “Low loss PCF by reduction of hole surface imperfection”, ECOC 2007, PD 2.1 (2007)). For example, K. Tajima, “Low loss PCF by reduction of hole surface imperfection”, ECOC 2007, PD 2.1 (2007) discloses a holey fiber that can reduce a transmission loss as low as about 0.18 dB/km at a wavelength of 1.55 micrometers. As just described, the broadband optical transmission using a holey fiber is a technology having a sufficient potential to be practically used in the future.
Characteristics of a holey fiber are mainly determined by a ratio d/Λ, which is a ratio of a diameter d of a hole to a distance Λ between adjacent holes. M. Koshiba and K. Saitoh, “Applicability of classical optical fiber theories to holey fibers”, Opt. Lett., vol. 29, no. 15, pp. 1739-1741 (2004) discloses that a holey fiber having holes arranged in a form of triangular lattice can realize a single-mode transmission at all wavelengths by setting d/Λ equal to or less than 0.43. The characteristic of being able to realize the single-mode transmission at all wavelengths is called the Endlessly Single-Mode (ESM) characteristic. If the single-mode transmission is realized in this manner, a faster optical transmission can be achieved. At the same time, a coupling of a light with a higher-order mode of the holey fiber can be prevented when the light is input into the holey fiber connected to another optical fiber and alike, thus preventing an increase of a connection loss.
As a type of the holey fiber, a multi-core holey fiber having a plurality of cores arranged separately from each other is disclosed (see International Publication No. WO 2006/100488 Pamphlet). Because the multi-core holey fiber can transmit a different optical signal through each of the cores, for example, it is considered to enable an ultra-high capacity transmission by way of a space division multiplexing (SDM) transmission.
However, with the conventional holey fiber, both an ordinary holey fiber having a single core and a multi-core holey fiber having a plurality of cores have a problem that a bending loss sharply increases particularly at the short-wavelength side as an operation wavelength band increases.
For example, the holey fiber disclosed in K. Ieda, K. Kurokawa, K. Tajima, and K. Nakajima, “Visible to infrared high-speed WDM transmission over PCF,” IEICE Electron. Express, vol. 4, no. 12, pp. 375-379 (2007) shows that a bending loss occurred when the fiber is wound ten times in a radius of 15 millimeters is 0.1 dB at a wavelength of 658 nanometers. However, when the inventors of the present invention experimented using a finite element method (FEM) simulation with the parameters (Λ=7.5 micrometers, d/Λ=0.5) disclosed in K. Ieda, K. Kurokawa, K. Tajima, and K. Nakajima, “Visible to infrared high-speed WDM transmission over PCF,” IEICE Electron. Express, vol. 4, no. 12, pp. 375-379 (2007), the bending loss of the fiber wound at a diameter of 20 millimeters is as high as 10 dB/m at the wavelength of 658 nanometers, which is considerably high. In addition, if d/Λ is reduced to achieve the ESM characteristic, the bending loss is considered to increase because an effective refractive index difference between the core and the cladding is also reduced.